Scanning is less reliable when the precise date of ovulation is not known.
Scanning twice, first at 15 days after ovulation (14-18 days after last covering), and again at 28-35 days after last covering, will reduce the risk of missing twin conceptions but will not eliminate it altogether.
Without accurate covering dates, early examinations are unreliable and may need repeating.
Diagnosis of a single pregnancy is not reliable at 14-18 days after covering.
Diagnosis of non-pregnancy is also unreliable at this stage, particularly if the ovulation date is unknown. For example, a stallion's sperm can remain viable for up to 6 days in the mare's genital tract. Therefore a mare scanned at 16 days may only be 10 days pregnant.
Even the most experienced operators can never be 100% certain that twins are not present.
If twins are present and the pregnancy is terminated by a prostaglandin injection, all the evidence shows that, if this is carried out by 30 days (i.e. before the initiation of endometrial cup development) the subsequent fertility for the mare is little different from that expected by mating her at any other normal oestrus. Terminations after 35 days reduce the likelihood of breeding in the same season.
Prostaglandin cannot be guaranteed to abolish pregnancy at any stage of gestation. A further examination 4-5 days later will help ascertain the outcome of treatment.
Older mares with endometrial cysts require more than one examination to confirm pregnancy and to rule out the possibility of multiple embryos. Serial examinations performed until at least 30 days after covering will assist in detecting the presence or absence of live foetuses.
A final examination is advised after 42 days because at this stage it is unlikely that the mare will lose the foetus.
It is accepted that there is a small, but inevitable risk of rectal damage when examining horses per rectum.
By understanding the limitations and risks of scanning for pregnancy in mares, it is less likely that your expectations of this procedure will be disappointed.
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